Planar laser-induced fluorescence (PLIF) technology is commonly used for flame structure diagnosis. For instance, OH hydroxyl (OH radical) and formaldehyde (CH₂O) groups in different flames have been extensively measured using the PLIF technology. Generally, the OH radical and CH₂O mainly exist in high- and low-temperature oxidation zones of carbon fuel flames, respectively. The heat release rate (HRR) of flames can also be measured indirectly by calculating [OH]×[CH₂O] after obtaining the OH-PLIF and CH₂O-PLIF images. However, most previous studies have focused on the HRR distribution measurement in turbulent flames with light fuels (e.g., methane) based on the PLIF technology and the measurement of flame thickness has received limited attention. In this study, the PLIF technology is used to measure the HRR and flame thickness of n-butane/air laminar flames under varied equivalence ratios and inlet flow velocities. Moreover, the obtained results are compared with those based on CH* chemiluminescence imaging. Because the oxidation reaction process of n-butane is similar to that of some large molecular hydrocarbon fuels, the findings of this study can provide quantitatively experimental data for further understanding the combustion characteristics of n-butane and large molecular hydrocarbon fuels.

In the experiment, a coflow burner is used to generate n-butane/air jet flames for optical diagnosis. The inner diameter of the burner center tube for supplying n-butane/air mixtures is 4.5 mm, and the diameter of the sintered bronze plug plate of the burner for supplying the accompanying air is 60 mm. The equivalence ratios of the mixtures are 1.1-1.5. The mixture flow velocity is varied from 1.6 to 2.0 m/s with the Reynolds number Re=497-636. The PLIF technology (LaVision Inc.) is employed to measure the flame structure. The OH-PLIF and CH₂O-PLIF measurements are performed using excitation wavelengths near 283 and 355 nm, respectively (~6.5 and 260 mJ/pulse, respectively, @ 10 Hz). All fluorescence signals are measured using an ICCD camera equipped with a UV lens and filters. The detected wavelengths of the OH and CH₂O fluorescence signals are ±310 and >390 nm, respectively, in the visible light wavelength range. Moreover, a digital camera and the ICCD combined with the CH* filter are used to record the flames in the visible wavelength range and CH* chemiluminescence, respectively. In data processing, 100 images of OH-PLIF and CH₂O-PLIF fluorescence signals are averaged and exported using the DaVis software by subtracting the background. Using the MATLAB program, the normalized [OH]×[CH₂O] images are obtained to represent the HRR distribution. Finally, the HRR and flame thickness of the n-butane/air coflow jet flames obtained using the aforementioned optical measurement methods are analyzed.

Based on the CH* chemiluminescence imaging, the OH-PLIF and CH₂O-PLIF images can clearly indicate the flamefront structure. At mixture equivalence ratios of 1.3 and 1.5, the open-tip structure of n-butane/air jet flames appears owing to the thermal diffusive instability. However, in the CH₂O-PLIF images, no such open-tip structure is observed (Fig. 2). The HRR position marked by [OH]×[CH₂O] is observed between the OH-PLIF and CH₂O-PLIF fluorescence signals. Furthermore, the HRR distribution profile of OH is similar to that of CH₂O and the HRR distribution of OH fairly overlaps with that of CH₂O. With increasing mixture flow velocity, the overlap zone between the HRR distributions of OH and CH₂O increases (Fig. 3). In the normal flamefront direction, the HRR distribution represented by [OH]×[CH₂O] and CH* shows a single-peak-value curve. Moreover, the peak-value positions of [OH]×[CH₂O] and CH* are consistent (Fig. 5). This confirms that the [OH]×[CH₂O] distribution measured using the PLIF technology can be used to indicate the HRR of n-butane flames. The thicknesses of the flames defined by the distance between the peak values of CH₂O (low-temperature reaction) and OH (high-temperature reaction) are 1.5-3.5 mm, considerably larger than the full width at half maximum (FWHM) of HRR in the CH* and [OH]×[CH₂O] images. The flame thicknesses are 0.3-1.0 mm using CH* and [OH]×[CH₂O], consistent with the theoretical results (Fig. 7).

Based on the PLIF and chemiluminescence imaging technologies, the structure of the n-butane/air coflow jet flames is measured under normal temperature and pressure conditions. The HRR distribution is obtained by calculating [OH]×[CH₂O] based on the OH-PLIF and CH₂O-PLIF images in the axis section, and the thickness of flames is obtained. The OH-PLIF images clearly show the open-tip structure at the top of the flame caused by the thermal diffusive instability. However, a strong CH₂O-PLIF signal is detected in the area, indicating a strong low-temperature oxidation reaction. The HRR distribution in the flame is located between the peaks of CH₂O and OH radical distributions, and the HRR distribution zone obtained using PLIF is slightly narrower than that obtained using the CH* chemiluminescence imaging. The equivalent ratio considerably influences the peak-value position of the HRR, while a weak influence of the flow rate is observed under low Reynolds number conditions. This is because the OH-PLIF signal shows a considerable shift with an increase in the mixture equivalence ratio. The flame thickness obtained based on the distance between the OH radical and CH₂O peak value positions is obviously greater than the FWHM of the HRR in the [OH]×[CH₂O] and CH* chemiluminescence images.